Buffer layer for thin film structures

ABSTRACT

A composite structure including a base substrate and a layer of a mixture of strontium titanate and strontium ruthenate is provided. A superconducting article can include a composite structure including an outermost layer of magnesium oxide, a buffer layer of strontium titanate or a mixture of strontium titanate and strontium ruthenate and a top-layer of a superconducting material such as YBCO upon the buffer layer.

STATEMENT REGARDING FEDERAL RIGHTS

This invention was made with government support under Contract No.W-7405-ENG-36 awarded by the U.S. Department of Energy. The governmenthas certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to a buffer layer for thin filmstructures. Among such thin film structures are included superconductingstructures, ferroelectric structures, photovoltaic structures andothers. The superconducting structures are those based upon YBa₂Cu₃O₇(YBCO) as the superconducter with a substrate of, e.g., single crystalsubstrates such as lanthanum aluminum oxide, aluminum oxide, magnesiumoxide and the like, and polycrystalline substrates such as nickel-basedalloys. The buffer layer is employed as a layer upon selectedsubstrates. In the superconducting structures commonly referred to ascoated conductors, the buffer layer is employed between and a YBCOsuperconducting layer and a base metal substrate having a magnesiumoxide coated substrate, the magnesium oxide layer deposited in an ionbeam assisted deposition. For example, high temperature superconductingfilms of YBCO can be grown epitaxially on epitaxial structures includingthe present buffer layer and provide high J_(c)'s and I_(c)'s.

BACKGROUND OF THE INVENTION

The preparation of oriented layers of crystallographically materials iscritical in a number of technical areas. For example, high temperaturesuperconductors often employ epitaxially oriented layers of, e.g., YBCOto achieve desired properties. Similarly, epitaxially oriented filmssuch as conductive oxides, ferroelectric, ferromagnetic, piezoelectric,insulating, and semiconductive materials are desired in the areas ofmicroelectric and opto-electric devices. To achieve the desiredcrystallographic orientation, e.g., epitaxial orientation, of materialssuch as YBCO, SrRuO₃, PZT, Ba_(1−x)Sr_(x)TiO₃, La_(0.7)Sr_(0.3)MnO₃, andSi, extensive work has focused on the underlying structure or substrateonto which the oriented layers are deposited.

Stronium ruthenate (SrRuO₃ or SRO) is a conductive metal oxide materialand it has been used as an electrode in ferroelectric capacitorapplications. Epitaxial SrRuO₃ thin films have been previously depositedupon stronium titanate (SrTiO₃ or STO) substrates (see, Chen et al.,“Epitaxial SrRuO₃ thin films on (001) SrTiO₃,” Appl. Phys. Lett., 711047 (1997). Strontium titanate is a dielectric oxide and has beenpreviously described as a suitable base substrate, e.g., in U.S. Pat.No. 6,541,136. Stronium titanate has also been previously described as abuffer material in superconductive structures, e.g., in U.S. Pat. No.6,562,761 wherein strontium titanate is suggested as a buffer layer onselected metal substrates and selected single crystal substrates, and inother microelectronic structures, e.g., in U.S. Pat. No. 5,471,364wherein strontium titanate is suggested as a buffer layer having amoderate dielectric constant.

Despite the many prior uses of both stronium ruthenate and stroniumtitanate, the use of stronium titanate alone or the use of a mixture ofstronium ruthenate and stronium titanate as a buffer layer has not beenpreviously described upon an IBAD-magnesium oxide layer in a coatedconductor superconducting structure. Additionally, the use of a mixtureof stronium ruthenate and stronium titanate as a buffer layer has notbeen previously described as a layer in other electronic applicationssuch as ferroelectric, ferromagnetic, piezoelectric and the like.

It is an object of the present invention to provide composite structuresincluding a buffer layer of strontium titanate or a mixture of strontiumtitanate and strontium ruthenate (SrTi_(x)Ru_(1−x)O₃ where 0<x≦1)directly on a magnesium oxide interlayer where the buffer layer issuitable for subsequent growth of a thin film layer, e.g., asuperconducting layer, thereon.

It is another object of the present invention to provide compositestructures including a buffer layer of a mixture of strontium titanateand strontium ruthenate (SrTi_(x)Ru_(1−x)O₃ where 0<x<1) where thebuffer layer is suitable for subsequent growth of a thin film layerthereon.

SUMMARY OF THE INVENTION

To achieve the foregoing and other objects, and in accordance with thepurposes of the present invention, as embodied and broadly describedherein, the present invention provides a composite structure including abase substrate, and a buffer layer of SrTi_(x)Ru_(1−x)O₃ where 0<x<1thereon the base substrate. Such a composite structure can serve as asupport for deposition of subsequent high quality epitaxial thin filmlayers thereon. Such thin film layers can be of materials such as hightemperature superconducting materials, e.g., YBCO and the like, such asferroelectric materials, e.g., barium strontium titanate, strontiumbismuth tantalate and the like, such as ferromagnetic materials, e.g.,lanthanum strontium manganate, lanthanum calcium manganate and the like,such as piezoelectric materials, e.g., lead zirconium titanate and thelike, and other such materials, e.g., yttrium manganate, bismuth ferriteor bismuth manganate.

In one embodiment, the present invention provides a composite structureincluding a base metallic substrate having a layer of magnesium oxidethereon, and a buffer layer of SrTi_(x)Ru_(1−x)O₃ where 0<x≦1 thereonthe layer of magnesium oxide. In a preferred embodiment, the magnesiumoxide layer is deposited by ion-beam-assisted deposition. Such acomposite structure can serve as a support for deposition of subsequenthigh quality epitaxial thin film layers of high temperaturesuperconducting materials.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a graph plotting critical current density (in MA/cm²) for1.5 micron thick YBCO coatings upon various SrTi_(x)Ru_(1−x)O₃ where0<x≦1 buffer layers in accordance with the present invention.

FIG. 2 shows a graph plotting critical current density (in MA/cm²) for1.5 micron thick YBCO coatings upon a strontium titanate(SrTi_(x)Ru_(1−x)O₃ where x=1) buffer layer deposited at varioustemperatures.

DETAILED DESCRIPTION

The present invention is concerned with buffer layers ofSrTi_(x)Ru_(1−x)O₃ where 0<x<1 for subsequent deposition of thin filmmaterials and composite structures for subsequent growth of thin filmlayers thereon. The present invention is further concerned withcomposite structures including a buffer layer of SrTi_(x)Ru_(1−x)O₃where 0<x≦1 on a base metllic substrate including a layer of magnesiumoxide thereon. These composite structures can be subsequently coatedwith superconducting materials, e.g., YBCO, as an epitaxial thin filmmaterial so as to form superconducting articles. High temperaturesuperconducting films can be grown epitaxially on such compositestructures and yield high J_(c)'s and I_(c)'s. Such high temperaturesuperconducting films can be prepared in the form of wires or tapes.

In the present invention, an initial or base substrate can be anypolycrystalline material such as polycrystalline metals orpolycrystalline ceramics or can be a single crystal base substrate suchas lanthanum aluminum oxide, aluminum oxide, magnesium oxide and thelike. Also, the initial or base substrate can be an amorphous substratesuch as silica or glass. In one embodiment, the base substrate can be apolycrystalline metal such as metal alloys. Nickel-based alloys such asvarious Hastelloy metals, Haynes metals and Inconel metals are useful asthe base substrate. Iron-based substrates such as steels and stainlesssteels may be used as the base substrate. Copper-based substrates suchas copper-beryllium alloys may also be useful as the base substrate. Inone embodiment, the base substrate can be a polycrystalline ceramic suchas polycrystalline aluminum oxide, polycrystalline yttria-stabilizedzirconia (YSZ), forsterite, yttrium-iron-garnet (YIG), silica and thelike.

The ultimate application can determine the selection of the material forthe base substrate. For example, the selection of the base substrate onwhich subsequent superconducting material (e.g., YBCO) is deposited canallow for the resultant article to be flexible whereby superconductingarticles (e.g., coils, motors or magnets) can be shaped. Thus, forsuperconducting applications requiring flexible substrates, the basesubstrate is a polycrystalline metal as these materials are generallyflexible, i.e., they can be shaped. For other applications, the basesubstrate on which other oriented materials are deposited may bepolycrystalline ceramics, either flexible or non-flexible. For stillother applications, the base substrate may be a single crystal substratesuch as magnesium oxide, lanthanum aluminate, or aluminum oxide.

Some base substrates can include a layer of a metal oxide or metaloxynitride material deposited upon an underlying substrate material.Such layers can be crystalline or amorphous. By “crystalline” is meantthat the atoms of the material have order over various length scales oflonger than a few nanometers. By “order” is meant that the atoms have adefinite pattern referred to as a lattice structure.

In some instances, the base substrate can include an oriented cubicoxide layer of, e.g., MgO, preferably deposited by ion-beam-assisteddeposition (IBAD) over one or more underlying materials. For example, ametal oxide or metal oxynitride material layer can be deposited on anunderlying material including a nickel based alloy coated with a firstlayer of aluminum oxide and a next layer of yttrium oxide, thedepositions carried out by pulsed laser deposition, e-beam evaporation,sputtering or by any other suitable means. Such a metal oxide or metaloxynitride material layer is generally deposited at temperatures ofgenerally about 100° C. Such a metal oxide or metal oxynitride materiallayer is typically from about 50 angstroms to about 1000 angstroms inthickness, preferably from about 50 angstroms to about 200 angstroms.Among the metal oxide or metal oxynitride materials suitable as thelayer are included yttrium oxide (Y₂O₃), aluminum oxynitride (AION),erbium oxide (Er₂O₃), yttria-stabilized zirconia (YSZ), cerium oxide(CeO₂), europium oxide, nickel aluminate spinel (NiAl₂O₄), and bariumzirconate (BaZrO₃). Preferably, the layer of metal oxide or metaloxynitride material is yttrium oxide, aluminum oxynitride, erbium oxideor yttria-stabilized zirconia and more preferably is yttrium oxide orerbium oxide.

In some instances, an underlying substrate material can include a firstcoating layer, i.e., an inert oxide material layer, upon the underlyingsubstrate material which is polished thereafter so as to have thedesired smoothness for subsequent depositions. Such an inert oxidematerial layer can also serve as a barrier layer to reduce or eliminatemigration of ions from the base substrate, particularly frompolycrystalline metal substrates, to other layers.

By “inert” is meant that a material does not react, or has negligiblereaction, with other material within the base substrate or with anysubsequently deposited materials. Examples of suitable inert oxidematerials include aluminum oxide (Al₂O₃), erbium oxide (Er₂O₃), yttriumoxide (Y₂O₃), and yttria-stabilized zirconia (YSZ). A preferred inertoxide material is aluminum oxide. Such an inert oxide layer can bedeposited on the base substrate by pulsed laser deposition, e-beamevaporation, sputtering or by any other suitable means. Such a layer isgenerally deposited at about room temperature.

In another embodiment of the invention, a single layer of erbium oxideis used as both an inert layer and a nucleation layer. With the use oferbium oxide as the crystalline layer onto which the MgO is deposited,there is generally no need to consider any other inert oxide layer uponthe base substrate. Such a crystalline erbium oxide layer can bechemically mechanically polished if desired.

Among oriented cubic oxide materials that can be included within thebase substrate structure can be included, e.g., magnesium oxide, calciumoxide, strontium oxide, zirconium oxide, barium oxide, europium oxide,samarium oxide and other materials such as described in U.S. Pat. No.6,190,752 by Do et al. Preferably, the layer of oriented cubic oxidematerial having a rock-salt-like structure is a magnesium oxide layer.Such a MgO layer is preferably deposited by electron beam evaporationwith an ion beam assist. The MgO layer in the ion-beam-assisteddeposition is typically sputtered from a magnesiun oxide target. Anion-assisted, electron-beam evaporation system similar to that describedby Wang et al., App. Phys. Lett., vol. 71, no. 20, pp. 2955–2957 (1997),can be used to deposit such a MgO film. Alternatively, a dual-ion-beamsputtering system similar to that described by Iijima et al., IEEETrans. Appl. Super., vol. 3, no. 1, pp. 1510 (1993), can be used todeposit such a MgO film. Generally, the substrate normal to ion-assistbeam angle is 45±3°.

An additional thin homo-epitaxial layer of MgO can be optionallydeposited as a layer on some base substrates, especially those includingan IBAD layer of magnesium oxide. Such an additional thin homo-epitaxiallayer of MgO can be deposited by a process such as electron beam ormagnetron sputter deposition. This thin homo-epitaxial layer cangenerally be about 50 angstroms to 1000 angstroms, prefereably 100angstroms to 500 angstroms in thickness.

As some base substrates can have a rough surface, such surfaces can bemechanically polished, electrochemically polished or chemicallymechanically polished to provide a smoother surface for optimization ofproperties in subsequently deposited layers. Electrochemical polishingof metallic substrates in an acid bath as described by Kreiskott et al.,Supercond. Sci. Technol., vol. 16, pp. 613–616 (2003), is generallypreferred.

In one embodiment of the present invention, the high temperaturesuperconducting (HTS) material is generally YBCO, e.g., YBa₂Cu₃O_(7−δ),Y₂Ba₄Cu₇O_(14+x), or YBa₂Cu₄O₈, although other minor variations of thisbasic superconducting material, such as use of other rare earth metalsas a substitute for some or all of the yttrium, may also be used. Amixture of the rare earth metal europium with yttrium may be onepreferred combination. Other superconducting materials such as bismuthand thallium based superconductor materials may also be employed.YBa₂Cu₃O_(7−δ) is generally preferred as the superconducting material.

A high temperature superconducting (HTS) layer, e.g., a YBCO layer, canbe deposited, e.g., by pulsed laser deposition or by methods such asevaporation including coevaporation, e-beam evaporation and activatedreactive evaporation, sputtering including magnetron sputtering, ionbeam sputtering and ion assisted sputtering, cathodic arc deposition,chemical vapor deposition, organometallic chemical vapor deposition,plasma enhanced chemical vapor deposition, molecular beam epitaxy, asol-gel process, liquid phase epitaxy and the like.

The composite structure provided in accordance with the presentinvention includes a base substrate and a layer of a mixture ofstrontium titanate and strontium ruthenate. Such a composite structureis useful for subsequent deposition of thin film layers of, e.g., YBCO.Optionally, other thin films can be deposited from a material selectedfrom the group consisting of superconductors, including high temperaturesuperconductors, semiconductors, photovoltaic materials, magneticmaterials, ferroelectric materials, ferromagnetic materials,piezoelectric materials, insulating materials, conductive materials andprecursors of superconductors or high temperature superconductors. Thecomposite structure is especially preferred for subsequent deposition ofhigh temperature superconductor materials.

Optionally within the present invention, one or more buffer layers canbe deposited so that they are between the composite structure (i.e., thebase substrate and the layer of a mixture of strontium titanate andstrontium ruthenate) and any subsequent high temperature superconductinglayer such as YBCO. The buffer layers can provide assistance in latticematching. Such so-called “buffer layers” should have good “structuralcompatibility” between the layer of strontium titanate or the layer of amixture of strontium titanate and strontium ruthenate and subsequentlydeposited superconducting materials, e.g., YBCO, and should have goodchemical compatibility with adjacent layers. By “chemical compatibility”is meant that the buffer layer does not undergo property-degradingchemical interactions, or has minimal interactions if at all, with theadjacent layers. By “structural compatibility” is meant that the bufferlayer has a substantially similar lattice structure with anysubsequently deposited material, e.g., superconductive material.

In one embodiment of the present invention, where the base substrateincludes a magnesium oxide layer thereon, preferably such a magnesiumoxide layer deposited by IBAD, the buffer layer is SrTi_(x)Ru_(1−x)O₃where 0<x≦1, i.e., the buffer layer can be a mixture of strontiumtitanate and strontium ruthenate or can be strontium titanate alone. Ingeneral, where mixtureds of strontium titanate and strontium ruthenateare used, the mixture can include a wide range of ratios. In one set ofexperiments, SrTi_(x)Ru_(1−x)O₃ where x is 0.5, was found to provide thehighest critical current density (J_(c)) for a high temperaturesuperconducting film of YBCO. The buffer layer of SrTi_(x)Ru_(1−x)O₃where 0<x≦1 is generally from about 200 angstroms to about 1500angstroms in thickness, preferably from about 400 angstroms to about 600angstroms in thickness. These buffer layers are generally deposited attemperatures of greater than about 600° C., preferably at temperaturesof from about 650° C. to about 850° C. For SrTi_(x)Ru_(1−x)O₃ where x=1,deposition temperatures of between about 800° C. and 825° C. have beenfound preferably for a base metallic substrate including a magnesiumoxide layer deposited by IBAD thereon.

In pulsed laser deposition, powder of the material to be deposited canbe initially pressed into a disk or pellet under high pressure,generally above about 1000 pounds per square inch (PSI) and the presseddisk then sintered in an oxygen atmosphere or an oxygen-containingatmosphere at temperatures of about 950° C. for at least about 1 hour,preferably from about 12 to about 24 hours. An apparatus suitable forpulsed laser deposition is shown in Appl. Phys. Lett. 56, 578 (1990),“Effects of Beam Parameters on Excimer Laser Deposition ofYBa₂Cu₃O_(7−δ)”, such description hereby incorporated by reference.

Suitable conditions for pulsed laser deposition include, e.g., thelaser, such as an excimer laser (20 nanoseconds (ns), 248 or 308nanometers (nm)), targeted upon a rotating pellet of the target materialat an incident angle of about 45°. The substrate can be mounted upon aheated holder rotated at about 0.5 rpm to minimize thickness variationsin the resultant film or coating, The substrate can be heated duringdeposition at temperatures from about 600° C. to about 950° C.,preferably from about 700° C. to about 850° C. An oxygen atmosphere offrom about 0.1 millitorr (mTorr) to about 500 mTorr, preferably fromabout 100 mTorr to about 400 mTorr, can be maintained within thedeposition chamber during the deposition. Distance between the substrateand the pellet can be from about 4 centimeters (cm) to about 10 cm.

The deposition rate of the film can be varied from about 0.1 angstromsper second (/s) to about 200/s b y changing the laser repetition ratefrom about 0.1 hertz (Hz) to about 200 Hz. Generally, the laser beam canhave dimensions of about 3 millimeters (mm) by 4 mm with an averageenergy density of from about 1 to 4 joules per square centimeter(J/cm²). After deposition, the films generally are cooled within anoxygen atmosphere of greater than about 100 Torr to room temperature.

Finally, where a subsequent layer of YBCO is deposited, it can generallybe deposited by pulsed laser deposition at a thickness of, e.g., about10,000 angstroms to 20,000 angstroms.

The present invention is more particularly described in the followingexamples which are intended as illustrative only, since numerousmodifications and variations will be apparent to those skilled in theart.

EXAMPLE 1

A nickel alloy substrate (Hastelloy C276) was ultrasonically cleaned insoap and water, rinsed with deionized water, rinsed with methanol andblown dry with filtered nitrogen. A layer of aluminum oxide about 350angstroms in thickness was deposited by ion beam sputter deposition.Onto this resultant article was deposited a layer of crystalline yttriumoxide (about 70 angstroms in thickness) also by ion beam sputterdeposition.

An ion-assisted, electron beam evaporation system similar to that ofWang et al., App. Phys. Lett., v. 71, no. 20, pp. 2955–2957 (1997), wasused to deposit a MgO film upon a flexible metal substrate of HastelloyC-276 or other nickel-based alloys. The ion source was manufactured byIon Tech, Inc. (Ft. Collins, Colo.) with a source geometry of 22 cm by2.5 cm. The substrate normal to ion-assist beam angle was 45±3°. The ionsource gas was argon. The ion source gas was introduced to a backgroundpartial pressure of about 1.0×10⁻⁶ Torr with a total pressure duringdeposition of about 1×10⁻⁴ Torr. The electron gun heated the MgO sourceto maintain a deposition rate of about 0.15 nm/sec. The ion-assist gunvoltage and current density were about 750 eV and 100 μA/cm²respectively.

Onto this resultant article was deposited a layer of MgO about 125angstroms in thickness using ion beam assisted electron beam depositionwith an ion assist gas of argon. Onto the IBAD-MgO layer was thendeposited a layer of homoepitaxial MgO (about 300 angstroms inthickness) by magnetron sputtering. Onto the top MgO layer was depositeda layer of a mixture of strontium titanate and strontium ruthenate (50atomic percent STO—50 atomic percent SRO) about 500 angstroms inthickness by pulsed laser deposition. Finally, a layer of YBCO about 1.4microns in thickness was then deposited on the STO-SRO by pulsed laserdeposition. The J_(c) (at 75 K) was measured as 2.3×10⁶ A/cm² using astandard four-point measurement. The projected I_(c) is 320 Amperesacross a sample 1 cm wide.

EXAMPLE 2

Additional nickel alloy substrates having similar coatings of aluminumoxide, yttrium oxide and magnesium oxide thereon were coated in themanner of Example 1 with various ratios of SrTi_(x)Ru_(1−x)O₃ where0<x≦1. Onto these substrates, a layer of YBCO was coated in the mannerof example 1. The resultant J_(c)'s were measured for the varioussubstrates and the data is shown in FIG. 1. The data suggest a preferredstrontium titanate (STO)-strontium ruthenate (SRO) ratio of about 50:50.

EXAMPLE 3

Nickel alloy substrates having similar coatings of aluminum oxide,yttrium oxide and magnesium oxide and SrTi_(x)Ru_(1−x)O₃ where 0<x≦1were overcoated with a second buffer layer of cerium oxide by pulsedlaser deposition. The resultant cerium oxide coatings were epitaxial innature.

EXAMPLE 4

Additional nickel alloy substrates having similar coatings of aluminumoxide, yttrium oxide and magnesium oxide thereon were coated in themanner of Example 1 with various ratios of SrTi_(x)Ru_(1−x)O₃ where x=1at various deposition temperatures. Onto these substrates, a layer ofYBCO was coated in the manner of example 1. The resultant J_(c)'s weremeasured for the various deposition temperatures (SrTiO₃) and the datais shown in FIG. 2. The data suggest a preferred deposition temperaturein the range of about 800° C. to about 825° C.

Although the present invention has been described with reference tospecific details, it is not intended that such details should beregarded as limitations upon the scope of the invention, except as andto the extent that they are included in the accompanying claims.

1. A superconducting article comprising: a base metallic substrateincluding one or more intermediate layers; a layer of magnesium oxidedeposited by ion-beam-assisted deposition thereon said one or moreintermediate layers of said base metallic substrate; a buffer layer of asingle phase SrTi_(x)Ru_(1−x)O₃ where 0<x≦1 thereon said layer ofmagnesium oxide; and, a top-layer of a high temperature superconductingmaterial upon the buffer layer of SrTi_(x)Ru_(1−x)O₃ where 0<x≦1.
 2. Thesuperconducting article of claim 1 wherein the high temperaturesuperconducting material is YBCO.
 3. The superconducting article ofclaim 2 further including a layer of a second buffer layer between saidlayer between said layer of SrTi_(x)Ru_(1−x)O₃ where 0<x<1 and saidtop-layer of a high temperature superconducting material.
 4. Thesuperconducting article of claim 3 wherein the second buffer layer iscerium oxide.
 5. A superconducting article comprising: a base metallicsubstrate including one or more intermediate layers; a layer ofmagnesium oxide deposited by ion-beam-assisted deposition thereon saidone or more intermediate layers of said base metallic substrate; abuffer layer of strontium titanate thereon said layer of magnesiumoxide; and, a top-layer of a high temperature superconducting materialof a thickness of from about 10,000 angstroms to about 20,000 angstromsupon the buffer layer of strontium titanate.
 6. A process of preparing asuperconducting article comprising: depositing a layer of magnesiumoxide by ion-beam-assisted deposition upon a base metallic substrateincluding one or more intermediate layers thereon; depositing a bufferlayer of single phase SrTi_(x)Ru_(1−x)O₃ where 0<x<1 thereon said layerof magnesium oxide a deposition temperatures between about 800° C. and825° C.; and, depositing a top-layer of a high temperaturesuperconducting material upon the buffer layer of SrTi_(x)Ru_(1−x)O₃where 0<x<1.